See this 45-minute CBC Nature of Things documentary on the effects of accumulating nutrients on the state of water quality in the Earth’s 10th largest freshwater lake:
Lake Winnipeg receives nutrient-rich flows from the Red, Assiniboine, and Winnipeg Rivers that drain a large continental agricultural basin.
The green sludge washing up on the shores of Lake Winnipeg is caused by high levels of chemical nutrients like phosphorous that have turned it into the most chlorophyll-polluted lake in the world. The nutrients come from its huge watershed – a million square kilometers. Phosphorous provides nutrition for the growth of algae – a natural process in healthy lakes. In the short-term, excess algae have led to bumper fish catches and happy fishermen. But when the algae die off, dead organic matter builds up, starts to decay and consumes dissolved oxygen. The lake then chokes from the bottom up – a condition known as hypoxia.